Various procedures for the preparation of isocyanates starting from aminic compounds via dialkylcarbonate are already described in literature for example, patent applications EQU WO 8805430; JP 2066-261; JP 2311-452
disclose a two-step procedure consisting in:
1) reacting a diamine with dimethylcarbonate in the presence of an alkaline alcoholate to produce urethane which is recovered from the mixture; PA1 2) the decomposition of the latter at 230.degree. C. in an organic solvent having a high boiling point in the presence of a metal and continuously removing the reaction products, by distillation. PA1 1) reacting a diamine with urea in the presence of dialkylcarbonate and alcohol to give the corresponding alkyl diurethane; PA1 2) subjecting the latter to partial vaporization and be then fed to a cracking reactor packed with a metallic filling with a low pressure loss which operates at a temperature which is higher than 300.degree. C.
Between 1) and 2) the catalyst is neutralized by treatment with acid, and then removed; the urethane is then purified by distillation.
This procedure has a problem in step 1) concerning the recovery of the dialkylurethane by distillation with the conventional methods, which is difficult to perform with good yields, and in step 2) various problems due to the purification of the liquid flows from the catalytic residues.
U.S. Pat. Nos. 4,596,678 and 4,596,679 also describe a two-step process consisting in:
Between 1) and 2) the urethane, which however contains synthetic by-products, which have a negative influence on the final result, is isolated.
U.S. Pat. No. 4,613,466 discloses a one-step process wherein dialkylurethane urethane is partially vaporized without decomposition and fed as such to a cracking reactor packed with a specific metallic filling having a catalytic function.
The products are then recovered by fractional condensation.
The above two-step process has, as already mentioned, the serious disadvantage of insufficient selectivity in the synthesis of the urethane during which are formed complex mixtures which are difficult to separate; the second step of the process and also the procedure described in the last patent have problems deriving from the presence of polymeric byproducts in the alkyl diurethane, which must be removed from the vaporization chamber by the liquid current discharged (partial evaporization) and from which it is difficult to recover the alkyl diurethane. This weighs heavily on the specific consumption of the process.